1. Field of the Invention
This disclosure is directed in general to hydraulic pumps and motors, and in particular to hydraulic machines having cylinder barrels rotatably coupled to valve plates.
2. Description of the Related Art
There is a class of hydraulic machines that employs a rotating barrel having a plurality of cylinders, and pistons reciprocating within the cylinders. The barrel is configured to rotate over a valve plate having inlet and outlet ports. The barrel rotates over the valve plate, and fluid passes into, and out of, the cylinders of the barrel. In a hydraulic pump, fluid is drawn into each cylinder from a low pressure inlet port and forced out of the cylinder to a high pressure outlet port. In a hydraulic motor, fluid from a high pressure inlet enters each cylinder in turn and vents to a low pressure outlet. Some machines, commonly referred to as pump/motors, are configured to operate as pumps or motors, according to how fluid is applied to the machine.
FIG. 1A shows a sectional view of a portion of a bent-axis pump/motor 100 according to known art. The pump/motor 100 includes a valve plate 102 and a cylinder barrel 104, having a plurality of cylinders 106, within which pistons 108 travel reciprocally. The pistons 108 each have a sliding seal engagement with walls of the respective cylinder 106, at first ends of the pistons. Each of the pistons 108 engages a respective socket formed in a thrust plate 110 at a second end thereof. Typically, bent-axis pump/motors are provided with an odd number of cylinders and pistons, usually seven or nine. In FIGS. 1A-1C cylinders 106 and pistons 108 are shown positioned at both the top and bottom of the barrel 104 simultaneously (which would not be the case in an actual machine employing an odd number of cylinders) for the purpose of illustrating the relative volumes of fluid constrained by the pistons 108 at the top and bottom of rotation.
The cylinder barrel 104 is configured to rotate around a first axis A with a face 114 of the cylinder barrel 104 slideably coupled to a face 116 of the valve plate 102. The thrust plate 110 rotates around an axis B, and is coupled to the rotating cylinder barrel 104 by a constant velocity joint, which is well known in the art, and is not shown in FIG. 1. Accordingly, the cylinder barrel 104 and the thrust plate 110 rotate at a common rate. The axis A is rotatable with reference to the axis B for the purpose of varying the displacement volume of the pump/motor 100. FIG. 1A shows the pump/motor 100 positioned at a moderate stroke angle. FIG. 1B shows the pump/motor 100 at a stroke angle of zero, wherein the axes A and B are coaxial, and wherein energy transfer is virtually zero. FIG. 1C shows the pump/motor 100 at a maximum stroke angle, which provides a maximum displacement of the pump/motor for a high degree of energy transfer.
As the cylinder barrel 104 rotates, each of the cylinders 106 follows a circular path. The uppermost point of that path is referred to as top-dead-center, indicated in FIGS. 1A-1C as TDC, while the lowermost point in the rotation is referred to as bottom-dead-center, indicated in FIGS. 1A-1C as BDC.
Referring to FIGS. 1B and 1C, it may be seen that when the pump/motor is at a minimum stroke angle, as shown in FIG. 1B, the fluid volume within the cylinders 106 at top-dead-center and bottom-dead-center is approximately equal. On the other hand, when the stroke angle is at a maximum, as shown in FIG. 1C, the volume of fluid within the cylinder 106 at bottom-dead-center is at a maximum, while the volume of fluid within the cylinder 106 at top-dead-center is at a minimum.
FIG. 2 shows the cylinder barrel 104 in a view indicated at lines 2-2 of FIG. 1A, the barrel face 114 being shown in plan view. A cylinder port 112 provides fluid communication from each of the cylinders 106 to the barrel face 114. The position of the cylinder 106 corresponding to each of the cylinder ports is shown in hidden lines.
FIG. 3 shows the valve plate 102 as seen from lines 3-3 of FIG. 1A, the surface 116 of the valve plate 102 being shown in plan view. TDC and BDC are also shown in FIG. 3, indicating the highest point of rotation, and lowest point of rotation, respectively.
Kidney ports 118, 119 are arranged respectively to the left and right of top-dead-center and bottom-dead-center of the valve plate 102. The kidney ports 118, 119 are configured to be differentially pressurized by high and low pressure fluid sources. As the cylinder barrel 104 rotates over the valve plate 102, each of the cylinder ports 112, shown in phantom lines in FIG. 3, is placed in fluid communication, alternately, with the kidney ports 118, 119.
The operation of the pump/motor 100, described with reference to FIGS. 1A-3, is well known in the art, and so will not be described in detail here. A more detailed description of the operation of a bent-axis pump/motor is described in U.S. patent application Ser. No. 10/379,992, which is incorporated herein by reference, in its entirety.
A problem common to many hydraulic machines incorporating features similar to those described herein occurs as each cylinder port traverses from contact with a first kidney port pressurized at a first pressure, to a second kidney port pressurized at a second pressure. For example, in a case where the pump/motor 100 is functioning as a motor, and wherein the kidney port 118 is pressurized at a high pressure, while the kidney port 119 is pressurized at a low pressure, the cylinder barrel 104 will rotate over the valve plate 102 in a counterclockwise direction R, as viewed in FIG. 3.
As each cylinder port 112 rotates over the kidney port 118 at the end closest to top-dead-center, pressurized fluid from the kidney port 118 will enter the cylinder 106 via the cylinder port 112. The pressurized fluid will drive the piston 108 outward in the cylinder 106, against the thrust plate 110, causing the barrel 104 and thrust plate 110 to rotate in the counterclockwise direction. As each piston 106 leaves the high pressure kidney port 118 at the end closest to the bottom-dead-center of the device, fluid within the cylinder 106 is maintained at the pressure of the high pressure fluid source coupled to the kidney port 118. At the moment that the cylinder port 112 begins to cross over onto the kidney port 119 at its end closest to the bottom-dead-center, a sudden drop in fluid pressure is realized within the cylinder 106, as the pressure within the cylinder is vented to the kidney port 119, which is pressurized at a low pressure. This sudden venting causes a pressure pulse in the pump/motor 100. A second pressure pulse occurs at the top of the cycle, as each of the cylinders 106, pressurized at the low pressure of the kidney port 119, begins to cross onto the kidney port 118 near top-dead-center, at which point each cylinder 106 is suddenly pressurized at the high pressure of kidney port 118.
Because most hydraulic machines are manufactured with an odd number of cylinders, the pressurizing pulses at the leading edge of the kidney port 118 and the depressurizing pulses at the leading edge of kidney port 119 occur alternately, with one pressurizing pulse and one depressurizing pulse occurring for each cylinder in each cycle of rotation. Accordingly, in a hydraulic machine such as pump/motor 100, having seven cylinders, there will be fourteen high energy pressure pulses per revolution. These pressure pulses are experienced as vibration in the pump/motor 100, as well as noise at a pitch corresponding to the frequency of pressure pulses.
Additionally, in known systems, when a cylinder port crosses into fluid communication with one of the kidney ports, there is an energy cost associated with bringing the corresponding cylinder to the pressure of the respective kidney port. For example, with reference to FIG. 3, as cylinder port 112a crosses the threshold of kidney port 118, the low pressure within the corresponding cylinder is brought up to the high pressure of the kidney port 118, which requires energy. On the other hand, as a cylinder port crosses bottom-dead-center and crosses over the threshold of the kidney port 119, the energy represented by the pressure within the corresponding cylinder is lost as that pressure is vented into the low pressure kidney port 119.
There are many known methods for reducing or smoothing the pressure pulses that occur as each cylinder transitions from one pressure to another. However, in each of these cases the energy losses described above still occur. One such scheme is described with reference to U.S. Pat. No. 6,186,748, issued to Umeda et al., which is incorporated herein by reference, in its entirety.